1. Field of the Invention
The present invention relates to a fine-tuning circuit for the winding voltage of a transformer. In particular, this invention relates to a fine-tuning circuit that is used for adjusting the output voltage of a main transformer in a power switching and conversion device.
2. Description of the Related Art
Transformers comprise a primary coil and at least one secondary coil, and are used for converting electrical power. Generally, the primary coil of a transformer is connected with a primary circuit and the secondary coil is connected with a secondary circuit. Electrical power from the primary circuit is transmitted to the primary coil of the transformer. Then, the transformer converts the electrical power into magnetic force, and the magnetic force is transmitted to the primary side of the transformer via the iron core of the transformer and is converted into electrical power and outputted via the secondary coil of the transformer. The electrical power on the secondary coil, such as the current or voltage, is related to the number of turns of the coil. The desired voltage or current is outputted from the secondary coil by adjusting the number of turns of the primary coil and the secondary coil of the transformer. Therefore, a variety of output voltages are available by adjusting the number of turns of the primary coil and a plurality of secondary coils of the transformer.
Nowadays, electronic circuits usually require a high power rate and a low voltage output. The secondary coil of the transformer cannot have too many turns, because of the problem of turns granularity. An engineer usually adjusts the number of turns of the coil to change the output voltage. However, this method cannot fully provide the desired voltage. For example, when a circuit needs to be 8V and 5V, 5V can be outputted if the number of the turns of the secondary coil of the transformer is 2. Under this condition, 7.5V is outputted when the number of the turns of the secondary coil of the transformer is 3 or 10V is outputted when the number of the turns of the secondary coil of the transformer is 4. The output voltage does not equal 8V. Therefore, the engineer needs to use a secondary coil with 3.2 turns to output the desired 8V. It can be implemented in a transitional transformer and a special transformer is needed. However, the special transformer has drawbacks, such as it is large in size, expensive, and needs special electric-magnetic components that are not easily obtained, etc.
Reference is made to FIG. 4, which is a schematic diagram of U.S. Pat. No. 6,348,848 that discloses a structure of a transformer having fractional-turn winding. An alternative solution for this problem is a coil with an exact number of turns (meaning, for example, 2, 3 or 4 turns, as opposed to 2.1, 3.3 or 4.7 turns) adopted and a resistor element, such as a linear constant voltage regulator, or voltage-time eating means, such as a saturable reactor, adopted to lower the voltage. However, both the resistor element and the voltage-time eating means suffer heat problems and lose power in the process. FIG. 5 is a circuit diagram of the power supply disclosed in U.S. Pat. No. 6,735,094. The power supply uses the saturable reactors Lsat3 and Lsat4 to lower the output voltages of secondary coils N3 and N4 of the transformers 1 and 2.
FIG. 6A is a circuit diagram of a linear constant voltage regulator that has the same effect as the resistor.
FIG. 6B is a circuit diagram of a switching constant voltage regulator that has the same effect as the resistor. The switching constant voltage regulator amends the output voltage and reduces power loss. Because the switching constant voltage regulator is expensive, large and complex it is not extensively used in electronic circuits.
FIG. 7 is a circuit diagram of a converter having multiple outputs. A circuit of the converter having multiple outputs was disclosed on Feb. 1, 2001 in EDN by Robert Bell. Robert Bell utilized a differential transformer to solve the problem of turns granularity of conventional transformers. The differential transformer T2 is an auxiliary transformer having a primary coil 23T and a secondary coil 10T. The primary coil 23T of the differential transformer T2 couples to the secondary coil 1T of the main transformer T1 in parallel. The secondary coil 10T of the differential transformer T2 couples to another secondary coil 1T of the main transformer T1 in serial. The secondary coil 10T of the differential transformer T2 is used for amending the output voltage of another secondary coil 1T of the main transformer T1.
However, the differential transformer T2 still has a problem. Although the primary coil 23T of the differential transformer T2 does not carry the output current, the primary coil 23T can have more turns. However, the secondary coil 10T cannot have more turns because the output current is already loaded on it. Therefore, the amended voltage is limited and the problem of turns granularity cannot be fully solved.
The differential transformer has another problem. The drop in voltage caused by a DC current flowing through the secondary coil 1T of the main transformer T1 makes the voltage-time product between the ends of the coil not equal zero. Because the primary coil 23T of the differential transformer T2 couples to the secondary coil 1T of the main transformer T1 in parallel, the non-zero voltage-time product nearly saturates the magnetic core of the differential transformer. In order to prevent saturation from occurring, the differential transformer T2 must have a larger resistor and a magnetic core of the primary coil 23T so that the volume of the differential transformer T2 is large.